The present invention relates to the measurement of fluid flow rates through piping and more particularly to an improved venturi arrangement therefor.
The measurement of steam and water flow rates in steam power plants (both conventional and nuclear) is a required operation at a variety of junctures in the various steam and water circuits comprising the steam power plant. Steam produced from a pressure vessel is taken through a steam line and applied to a steam turbine. Exhaust steam from the turbine is condensed and returned to the pressure vessel as feedwater. It is desirable to measure the flow rate of the steam through the main steam lines to the turbine as well as to place a restriction in the steam lines to limit the escape of steam and water from the pressure vessel during the period of time required for shut-off valves to close in the unlikely event that the steam lines become open. In a nuclear steam generating facility, this is known as a loss of coolant accident (LOCA) where a primary or secondary pipe break may occur in the piping system either outside or inside the dry well and the rate of loss of coolant exceed a rate of normal feedwater make-up. It is a mechanical and economic advantage if flow measurement and flow restriction both can be achieved utilizing the same device.
If a constriction is placed in a closed channel carrying a stream of fluid, there will be an increase in velocity, and hence an increase in kinetic energy, at the point of constriction. From an energy balance, as given by Bernoulli's Theorem, there must be a corresponding reduction in pressure. Rate of discharge from the constriction can be calculated knowing this pressure reduction, the area available for flow at the constriction, density of the fluid, and the coefficient of discharge. The latter is defined as the ratio of actual flow to the theoretical flow, and makes allowance for stream contraction and frictional effects. A standard, Herschel-type venturi meter consists of a short length of straight tubing connected at either end to the pipe by conical sections. The straight and conical sections should be joined by smooth curved surfaces for best results. From a variety of practical working equations known in the art, the flow of fluid through the venturi meter can be measured by a calibrated differential pressure meter connected to a pressure tap in the throat section of the venturi and to a line pressure tap preferably placed in the fluid line upstream of the venturi meter.
The venturi configuration set forth in U.S. Pat. No. 3,859,853 can be used to obtain good, accurate measurement of steam flow when the venturi is installed in a vertical piping configuration. The same arrangement also can be used for measuring recirculation water flow and feedwater flow entering reactor pressure vessels located at steam power plants. The circumferential welding of the venturi downstream section to the internal surface of the pipe, however, requires a cumbersome analysis to substantiate its pressure integrity during plant normal operation as well as during a LOCA. Another problem with such configuration is the requirement for in-service inspection (ISI) of any part welded to a primary pressure retaining component (i.e., main steam and recirculation piping) when the steam power plant utilizes nuclear fuel. Since this inner weld cannot be inspected from outside the piping, a detailed analysis is required to be performed in order to comply with the code requirements. Also, this weld technically is not a full-penetration weld in accordance with ASME-III Code Definition, Section XI. The venturi arrangement set forth in U.S. Pat. No. 3,889,537 addresses some of these problems, yet it still is not entirely satisfactory.